Transcript Chapter 9 Capacitors - National University of Kaohsiung

Chapter 9
Capacitors
Objectives
• Describe the basic structure and
characteristics of a capacitor
• Discuss various types of capacitors
• Analyze series capacitors
• Analyze parallel capacitors
• Describe capacitive dc switching circuits
• Describe capacitive ac circuits
Basics of a Capacitor
• In its simplest form, a capacitor is an electrical
device constructed of two parallel plates separated
by an insulating material called the dielectric
• In the neutral state, both plates have an equal
number of free electrons
• When a voltage source is connected to the
capacitor, electrons are removed from one plate and
an equal number are deposited on the other plate
• No electrons flow through the dielectric (insulator)
Basics of a Capacitor
• When the supply is removed from the capacitor, the
capacitor retains the stored charge
• The amount of charge that a capacitor can store per
volt across the plates is its capacitance (C)
• The unit of capacitance is the farad (F). One farad
is the amount of capacitance when one coulomb of
charge is stored with one volt across the plates
• Most capacitors in electronics work have
capacitance values of F (10-6 F) or F (10-12 F)
How a Capacitor Stores Energy
• A capacitor stores energy in the form of an electric
field that is established by the opposite charges on
the two plates
• A capacitor obeys Coulomb’s law:
A force exists between two point-source charges
that is directly proportional to the product of
the two charges and inversely proportional to
the square of the distance between the charges
Capacitor Ratings
• The voltage rating specifies the maximum dc
voltage that can be applied without risk of damage
to the device (breakdown or working voltage)
determined by the dielectric strength
• Temperature coefficient indicates the amount and
direction of a change of capacitance with
temperature
– positive coefficient means that capacitance increases
with increasing temp., while a negative coefficient
means capacitance decreases with increasing temp.
Characteristics of a Capacitor
• Capacitance is directly proportional to the physical
size of the plates as determined by the plate area
• Capacitance is inversely proportional to the
distance between the plates
• The measure of a material’s ability to establish an
electric field is called the dielectric constant ()
– Capacitance is directly proportional to the dielectric
constant
Fixed Capacitors
• Stacked-foil mica capacitors are made of alternate
layers of metal foil and thin sheets of mica
• Silver mica are formed by stacking mica sheets
with silver electrode material screened on them
Fixed Capacitors
• Ceramic dielectrics provide very high dielectric
constants, and relatively large capacitance in a
small physical size
• Capacitance ranges are from 1pF to 2.2F
Electrolytic Capacitors
• Electrolytic capacitors are polarized so that one
plate is positive, and the other negative
• They come in capacitance values from 1F to
200,000 F, with voltage ratings to 350 V
Electrolytic Capacitors
• Two common types of electrolytic
capacitors are Aluminum and Tantalum
electrolytics
• The voltage polarity of these devices must
be observed, as reversal of polarity will
usually result in complete destruction of the
capacitor
Variable Capacitors
• Variable capacitors are used in circuits when there
is a need to adjust the capacitance value
• Ceramic or mica is a common dielectric
• Capacitance is changed by plate separation
Capacitor Labeling
• Capacitors use several standard labeling methods;
we will consider a small ceramic capacitor:
– values marked as .001 or .01 have units of microfarads
– values marked as 50 or 330 have units of picofarads
– a value of 103 or 104 would be 10x103 (10,000 pF) or
10x104 (100,000 pF) respectively
– The units may be on the capacitor as pF or F (F may
be written a MF or MFD)
Series Capacitors
• When capacitors are connected in series, the total
capacitance is less than the smallest capacitance
value since the effective plate separation increases
1/CT = 1/C1 + 1/C2 + 1/C3 + … + 1/Cn
Parallel Capacitors
• The total parallel capacitance is the sum of all
capacitors in parallel
CT = C1 + C2 + C3 + … + Cn
Capacitors in DC Circuits
• A capacitor will charge up when it is
connected to a dc voltage source
• When a capacitor is fully charged, there is
no current
• There is no current through the dielectric of
the capacitor because the dielectric is an
insulating material
• A capacitor blocks constant dc
RC Time Constant
• The voltage across a capacitor cannot
change instantaneously because a finite time
is required to move charge from one point
to another (limited by circuit resistance)
• The time constant of a series RC circuit is a
time interval that equals the product of the
resistance and the capacitance
 = RC
Charging and Discharging
• The charging curve is an increasing exponential
• The discharging curve is a decreasing exponential
Transient time
• It takes 5 time constants to change the
voltage by 99% (charging or discharging),
this is called the transient time
Capacitors in ac Circuits
• The instantaneous capacitor current is equal to the
capacitance times the instantaneous rate of change
of the voltage across the capacitor
• This rate of change is a maximum positive when
the rising sine wave crosses zero
• This rate of change is a maximum negative when
the falling sine wave crosses zero
• The rate of change is zero at the maximum and
minimum of the sine wave
Capacitive Reactance, XC
• Capacitive reactance (XC) is the opposition to
sinusoidal current, expressed in ohms
• The rate of change of voltage is directly related to
frequency
• As the frequency increases, the rate of change of
voltage increases, and thus current ( i ) increases
• An increase in i means that there is less opposition
to current (XC is less)
• XC is inversely proportional to i and to frequency
Capacitive Reactance, XC
• The relationship between capacitive
reactance, capacitance and frequency is:
XC = 1/(2 f C)
where: XC is in ohms ()
f is in hertz (Hz)
C is in farads (F)
Analysis of Capacitive ac Circuit
• The current leads the
voltage by 90 in a
purely capacitive ac
circuit
Power in a Capacitor
• Energy is stored by the capacitor during a portion
of the voltage cycle; then the stored energy is
returned to the source during another portion of
the cycle
• Instantaneous power (p) is the product of v and i
• True power (Ptrue) is zero, since no energy is
consumed by the capacitor
• The rate at which a capacitor stores or returns
energy is called reactive power (Pr); units: (VAR)
Capacitor Applications
• Capacitors are used for filtering in power supplies
• Since capacitors do not pass dc, they are used for
dc blocking and ac coupling
• For power line decoupling, capacitors are
connected between the dc supply and ground, to
suppress unwanted voltage spikes that occur on
the dc supply voltage due to fast switching
• Capacitors are used to bypass an ac voltage around
a resistor without affecting the dc resistance
Capacitor Applications
• Capacitors are used in filters, to select one ac signal
with a certain specified frequency from a wide
range of signals with many different frequencies
– For example, the selection of one radio station and
rejecting the others
• Capacitors are used in timing circuits to generate
time delays, based on the RC time constant
• Dynamic memories used in computers are simply
very tiny capacitors used as a storage element
Summary
• A capacitor is composed of two parallel
conducting plates separated by a dielectric
insulator
• Energy is stored by a capacitor in the
electric field between the plates
• Capacitance is measured in units of farads
(F)
Summary
• Capacitance is directly proportional to the plate
area and inversely proportional to the plate
separation
• Dielectric constant is an indication of the ability of
a material to establish an electric field
• Dielectric strength is one factor that determines
the breakdown voltage of a capacitor
• A capacitor blocks constant dc
Summary
• The time constant for a series RC circuit is the
series resistance times the capacitance
• In an RC circuit, the voltage and current in a
charging or discharging capacitor make a 63%
change during each time-constant interval
• 5 time constants are required for a capacitor to
fully charge or to discharge fully. This is called
the transient time
• Charging and discharging are exponential curves
Summary
• Current leads voltage by 90 in a capacitor
• Capacitive reactance is the opposition to ac,
expressed in ohms
• XC is inversely proportional to frequency and
capacitance value
• The true power in a capacitor is zero; that is, there
is no energy loss in an ideal capacitor
– most capacitors have some small energy loss due to
leakage resistance